Cell cycle checkpoints ensure the integrity of the genome from one replicative cell cycle to the next. In the event of catastrophic damage to the genome, cells of multicellular eukaryotes can undergo apoptosis, presumably to eliminate them from the cell population and reduce the risk of propagating genetically unstable cells. Alternatively, cells may respond to DNA damage by undergoing a transient arrest of the cell cycle, which correlates with their ability to survive exposure to DNA damaging agents. This response requires the DNA damage checkpoint pathway;if compromised by mutation or drug treatment, cells will enter mitosis with damaged DNA and die. The fission yeast has been an extremely valuable system for identifying and characterizing components of the DNA damage checkpoint. Indeed, many proteins that are now known to function in the checkpoint pathway in mammalian cells were identified solely based on their sequence homology to proteins that were identified genetically and functionally in yeast. Thus, it is clear that the identification of proteins in yeast using the power of classical genetics is a valid and productive means of identifying and gaining insight into the function of mammalian counterparts. Experiments described in this proposal will continue to investigate the protein kinase Chk1, a key regulator of the checkpoint in eukaryotic cells. In addition, we will focus on a novel fission yeast protein, Msc1. Msc1 shares structural domain homology with mammalian RbBP2, a protein identified by virtue of its ability to bind the tumor suppressor protein Rb and with PLU-1, the product of a gene that is up regulated in breast cancer cells. Msc1 was cloned because it can compensate for the loss of function Chk1. The Msc1 protein is important for chromosome stability. Experiments described in this proposal aim to dissect the role of the Msc1 protein in fission yeast, to lay the groundwork for understanding the functions of the human homologues. When cells experience damage to their DNA, they delay cell cycle progression in order to repair the damage. Agents that damage DNA are used in therapy of cancer because they tend to preferentially kill cells that are rapidly dividing, a feature common to many types of cancer. Pathways that delay cell cycle progression are called checkpoint pathways and it is critical to understand how these pathways work, both to appreciate how DNA damage brings about cell death and to identify ways in which to enhance killing by chemotherapeutic agents. The fission yeast has been an extremely valuable system for identifying and characterizing components of the DNA damage checkpoint. Our studies make use of the excellent genetics of this organism to elucidate mechanisms that are universally important for governing cell division in the face of DNA damage.